In conventional working vehicles the drive power source is normally an engine. However, in circumstances in which an engine cannot be used, such as in an underground construction site, construction machinery with an electric motor as the power source is used (see for example Japanese Patent Application Laid-open No. 2004-225355). The power sources for these electric motors are a commercial power supply, or batteries on the construction machinery that are charged from a commercial power supply. Also, the operation of this type of electric motor is controlled using an inverter.